Method for driving an output stage by pulse-width modulation

FIELD AND BACKGROUND OF THE INVENTION

The invention relates to a method for driving an output stage by pulse-width modulation, which driving serves to supply a unit cooling the output stage during operation.

If electric motors are used to drive cooling units, in particular refrigerating machines of air conditioners or fans in motor vehicles, the cooling can also be used to cool the output stage. Various requirements have to be considered in the case of driving the output stage by pulse-width modulation (PWM)—inter alia with regard to electromagnetic compatibility and noise intrusions at frequencies below the auditory threshold. These requirements are, however, frequently opposed to a high edge steepness or to a low frequency of the pulses, which are desirable in order to achieve the lowest possible power loss in the output stage.

SUMMARY OF THE INVENTION

It is the object of the present invention to design a style of driving an output stage, which driving serves to supply a unit cooling the output stage during operation, in such a way that both the said ambient conditions are generally satisfied, and overloading of the output stage at high ambient temperatures owing to a high power loss is avoided.

According to the invention, this object is achieved by virtue of the fact that at a higher temperature of the output stage a style of driving is performed which results in a low power loss in the output stage, and that at a lower temperature of the output stage driving is performed in such a way that the power loss is higher.

The invention is based on the finding that by cooling during operation for the benefit of the ambient conditions it is possible to permit a higher power loss, whereas when there is no cooling, something which is substantially the case in a short run-up phase, it is possible to permit a somewhat more unfavorable environmental response.

A first embodiment of the invention reduces the power loss during the run-up phase by virtue of the fact that at a higher temperature driving is performed with pulses of higher edge steepness, and at a lower temperature it is performed with pulses of lower edge steepness, whereas a second embodiment reduces the power loss during the run-up phase by virtue of the fact that at a higher temperature driving is performed with a lower PWM carrier frequency than at a lower temperature. It is also possible for the two embodiments to be combined with one another.

Depending on the detail of the preconditions, it is possible to provide in the case of the method according to the invention that there is a gradual transition or that a switchover is made between two styles of driving.

A further refinement of the method according to the invention proceeds from the fact that the higher temperatures occur in a run-up phase, and therefore provide that the transition from a style of driving which is provided at a higher temperature to a style of driving for a lower temperature is performed under time control after switching on. As a result, there is no need either for a temperature sensor nor for a circuit evaluating the output signal thereof.

It is provided according to another embodiment that a selection is made between the styles of driving as a function of a measured temperature. Possible temperature increases outside the run-up phase are also taken into account in this case. Already existing temperature sensors can also be used, if appropriate.

BRIEF DESCRIPTION OF THE DRAWING

The invention permits numerous embodiments. One of these is illustrated diagrammatically in the drawing with the aid of a plurality of figures and is described below. In the drawing:

FIG. 1 shows an arrangement for carrying out the method according to the invention,

FIG. 2 shows pulse diagrams in the case of a first embodiment of the method according to the invention,

FIG. 3 shows pulse diagrams in the case of a second embodiment,

FIG. 4 shows the drive frequency as a function of time,

FIG. 5 shows the frequency as a function of temperature,

FIG. 6 shows the edge steepness (tR/F) as a function of temperature, and

FIG. 7 shows the edge steepness (tR/F) as a function of time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows as a block diagram in a greatly simplified form an output stage <b>1</b> which is driven by a control device <b>2</b> with pulse-width-modulated signals. Various signals required for controlling purposes, for example input/output signals or speed/temperature stipulations can be fed to the control device via the inputs <b>3</b> and <b>4</b>. The output stage <b>1</b> supplies a motor <b>5</b>, which drives a refrigerating unit <b>6</b>. The refrigerating unit is connected in a thermally conducting fashion to the output stage <b>1</b>, and this is symbolized by arrows. A temperature sensor <b>7</b> measures the temperature of the output stage <b>1</b> or of another point in the system which has a good correlation with the output stage temperature, and passes the measurement result to control device <b>2</b>.

In the first embodiment of the method according to the invention, according to FIG. 2<i>a</i>) the output stage <b>1</b> receives pulses with steep edges such that the power loss in the output stage <b>1</b> is so low that even at a higher temperature the permissible maximum temperature of the output stage transistors is not exceeded. Such high temperatures can also occur, for example, in the engine compartment when a motor vehicle is parked after being driven quickly and/or at high ambient temperatures. After a run-up phase, the output stage <b>1</b> is cooled by the refrigerating unit <b>6</b>. A changeover is then made to the pulses shown in FIG. 2<i>b</i>), although, because of their lower edge steepness, the said pulses result in a higher power loss in the output stage, but exhibit a weaker interfering effect in higher frequency ranges of the electromagnetic emission.

FIG. 3 shows an alternative, specifically switching over the carrier frequency, a lower frequency being provided in the run-up phase shown in FIG. 3<i>a</i>), with the result that the power loss caused by the finite edges is lower. Short-term noise emission is possible in return when the frequency is below 16 kHz. After the lower operating temperature has been reached, a switchover is made to a high frequency above the range of audibility with edges which are not excessively steep (FIG. 3<i>b</i>)).

FIG. 4 shows the switchover of the frequency after a prescribed time t_<b>0</b> from a lower to a higher value, as is illustrated by way of example in FIG. <b>3</b>.

FIG. 5 shows the frequency f as a function of the temperature theta, the frequency being high at lower temperatures and low at higher ones. FIG. 6 shows the edge steepness t_R/F as a function of temperature, and FIG. 7 shows it as a function of time.

Depending on the preconditions that are present, the transition can be performed continuously (continuous line) or in a plurality of stages or in one stage (dashed or dotted lines).

In many cases, the run-up time is very short and is, for example, 10<i>s </i>to 15<i>s </i>for air-conditioning compressors.